How do I rig a game asset for a state change? (Armoured/Normal/Damaged)
I'd say (without knowing fizzle about Unity) that your best shot would be shape keys, but...
If I had to do it with a bone in blender animating, I'd try driving object visibility or material transparency by a bone's position/rotation with a greater-than statement.
Working from my knowledge of the BGE which may or may not be similar to Unity (I have a DeLorion), I'd say you should have a script switch out the meshes based on the bone.
Cute tank BTW.